1. Field of the Invention
The present invention relates to a composition and a process for producing thin, solid, polymeric films, by liquid deposition on a substrate with subsequent electron beam curing. Both the liquid deposition and curing are done in a substantially oxygen-free atmosphere under vacuum conditions. The composition comprises one or more components, all of which components do not go into a gas phase under the vacuum conditions. The composition has an ethylenically unsaturated component composed of an ethylenically unsaturated methacrylate monomer, or a combination of an ethylenically unsaturated methacrylate monomer and an ethylenically unsaturated methacrylate oligomer. The ethylenically unsaturated component is polymerizable or crosslinkable by the application of sufficient electron beam radiation. The composition is substantially absent of ethylenically unsaturated acrylate components, polymerization initiators, and solvents.
2. Description of the Related Art
There is great commercial interest in applying protective and/or functional coatings over metalized film substrates directly inside of a vacuum chamber and curing them via electron beam irradiation. A benefit of electron beam curable compositions is that they are essentially completely solid and do not transfer into the gas or vapor phase under the vacuum. Applying solid curable coatings under vacuum is beneficial for coating uniformity and adhesion to non-oxidized metal surfaces. This is beneficial in comparison to applying electron beam curable coatings in air over oxidized metal surfaces.
Thin metallic and polymeric films add or promote desirable properties for particular applications. For example, foils used to preserve food need to have very low permeability to oxygen; the exterior surface of packaging material has to be capable of accepting printing inks; and packaging materials for electronic products also require a limited amount of conductivity to dissipate electrostatic charges. It is desirable and sometimes necessary to modify the physical properties of polymeric films to improve their suitability for the intended purpose. Preferably, the films are directly formed with a composition and molecular structure characterized by the desired properties. Thin films of metals and polymers are formed by deposition onto appropriate substrates by a variety of known processes, most notably through film formation by wet chemistry or vapor deposition. Chemical processes produce soluble thermoplastic as well as insoluble thermoset polymers and involve the use of solvents; thus, film formation is achieved through solvent diffusion and evaporation. As a result, these processes require relatively long residence times and the undesirable step of handling solvents.
Vapor deposition processes involve the evaporation of a liquid monomer in a vacuum chamber, its deposition onto a cold substrate, and subsequent polymerization by exposure to electron beam or ultraviolet radiation. U.S. Pat. Nos. 6,270,841 and 6,447,553 illustrate a liquid monomer from a supply reservoir which is atomized in a heated evaporator section of a vacuum deposition chamber where it flash vaporizes under vacuum. The resulting monomer vapor passes into a condensation section of the unit where it is vapor applied onto a substrate, condenses and forms a thin liquid film upon contact with the cold surface of the substrate. The liquid deposited film is then cured by exposure to an electron beam or ultraviolet radiation source. A problem with such a technique is that the vaporized composition coats much of the inside of the equipment inside the vacuum chamber, and then cures into an unwanted solid on the equipment when irradiated. Such unwanted solids are difficult to remove. In addition, these patents teach compositions which contain multifunctional acrylates which polymerize to solids almost instantaneously in an oxygen free atmosphere.
Traditionally, electron beam curable coatings are mixtures of acrylate functional pre-polymers, oligomers and monomers that can undergo free-radical polymerization under exposure to electron beam irradiation. Typically, electron beam free radical polymerization is inhibited by the presence of oxygen and therefore electron beam coatings must cure under a nitrogen blanket. The complete curing requires a substantial electron beam dose.
It is also known that methacrylate functional pre-polymers, oligomers and monomers can undergo free radical polymerization but at much slower rate than polymerization of acrylates, and they cannot be used in any known commercial application of electron beam curing technology. Most of acrylate functional compounds can quickly polymerize in oxygen free atmosphere, especially under high temperature conditions even without exposure to electron beam irradiation.
It was surprisingly found that methacrylate based compositions can actually retain their fluidity under the vacuum and elevated temperature conditions, avoiding premature polymerization. Even small quantity additions of acrylate functional groups quickly lead to loss of stability. According to the invention, a composition is formed which has an ethylenically unsaturated component composed of an ethylenically unsaturated methacrylate monomer, or a combination of an ethylenically unsaturated methacrylate monomer and an ethylenically unsaturated methacrylate oligomer. The ethylenically unsaturated component is polymerizable or crosslinkable by the application of a low dose of electron beam radiation. The composition is substantially absent of ethylenically unsaturated acrylate components, polymerization initiators, and solvents. The resulting cured composition on the substrate gives much improved moisture barrier, oxygen barrier scratch resistance, adhesion and printability properties. The process affords a complete curing, with a low voltage electron beam cure, and without oxygen which would oxidize metalized surfaces leading to brittleness and difficulty in printing.